Prior to reading your response, I thought I would try to recreate what you did; my Volt was fully charged with the blinking light and the estimated range showed 41miles. I powered on the Volt, turned off the auto headlights and yes, the charge light turned to solid.
I intended to allow this to charge for 1 hour and see what range difference it would make (don't have the test app, you have);
after about 20-30mins, I went back to garage and found the charge light blinking (blows the theory, at least in my case). I turned off the Volt and powered back on and the estimated range still showed 41 miles and interestingly enough, this time, my charge light continued to blink. There must be a maximum the battery will really MAX out at and possibly every Volt is a little different in that way.............maybe you got yourself a 'stronger' battery than me???
BTW, some time ago, I tried what I refer to as 'deep charge cycle' where the Volt is plugged in and shows fully charged with the blinking light; you simply unplug the Volt and then plug it back in without doing anything else and that showed the Volt charging again but not sure for how long..........my guess is probably approx. 20mins or when it thinks it has MAXed out (same as above).